1. Field of the Invention
The present invention relates to a capacitive physical quantity sensor for detecting a physical quantity and a method of diagnosing the same. For example, the sensor may be used as an acceleration sensor, in which a capacitance formed between a movable electrode and a fixed electrode varies, when a physical quantity is applied.
2. Description of Related Art
U.S. Pat. No. 6,483,322 (corresponding to JP-A-2002-40047) discloses an acceleration sensor having a self-diagnostic function, as a capacitive sensor for detecting a physical quantity (dynamic amount). When the sensor is in a self-diagnostic mode, a voltage is applied between a movable electrode and a fixed electrode in the sensor. Thereby, the movable electrode is forcibly displaced and made to oscillate. The sensor performs a self-diagnosis about an abnormality generation by detecting an output of an acceleration signal corresponding to the oscillation.
Specifically, the sensor includes a control circuit, which applies a voltage for driving the movable electrode in order to detect an acceleration and perform the self-diagnosis. The control circuit includes an input terminal and a status (STAT) terminal corresponding to an output terminal. A personal identification number (PIN) signal for instructing the self-diagnosis is input into the input terminal, the STAT terminal outputs an identification (ID) signal for indicating which acceleration sensor is performing the self-diagnosis.
When the PIN signal is input into the input terminal, the self-diagnosis is performed by applying the voltage between the movable electrode and the fixed electrode. FIG. 11 is a timing chart showing a self-diagnostic operation by a conventional acceleration sensor. As shown in FIG. 11, when the PIN signal is set to a high level (Hi), a self-diagnostic mode starts, and an output signal Go of the acceleration sensor is set to Hi. At the same time, the STAT terminal is set to Hi, and a self-diagnosis starts. Then, after the PIN signal is set to a low level (Lo), an ID signal is output from the STAT terminal to indicate which acceleration sensor is performing the self-diagnosis.
However, when the PIN signal is set to Hi by external noises, e.g., electromagnetic noises, the output signal Go of the acceleration sensor is set to Hi, while the acceleration sensor is not in the self-diagnostic mode. This signal may indicate that an acceleration is applied, and an apparatus operating based on the acceleration sensor may malfunction.